Traditional fast MRI sequences collect multiple echoes per excitation, each echo having a different echo time. In a train of spin echoes, each successive echo has a decreased signal. Thus, collecting a train of spin echoes with different echo times generally degrades resolution; the decrease in signal with each echo is analogous to an apodization function. For example, during the train of gradient echoes in echo planar imaging (EPI), image blurring cannot be avoided due to the relatively short effective transverse relaxation time (T2*); thus, EPI is not a suitable sequence for resolving small objects.
Pulse sequences using sequential excitation and/or refocusing of spins along the direction of an applied field gradient have been developed in the past. In their paper, “A Time Encoding Method for Single-Shot Imaging,” Meyerand et al. introduced a time-encoding sequence that uses a series of pulses to excite different slices, followed by a single 180° pulse to create spin echoes that are read out in opposite order. Magn Reson Med, 1995. 34: p. 618-622. Although this sequence produces a series of spin echoes, the echo time (TE) of the different echoes varies widely across the object. Further, in Meyerand sequences, fast gradient switching is needed between 90° pulses.
The Meyerand et al. technique is a multi-slice technique having gaps between slices. As can be appreciated, information is lost in the gaps between the slices.
Frydman et al. introduced a rapid MRI method which exploits a chirp pulse to spatially-encode one dimension instead of frequency encoding. Tal, A. and L. Frydman, Spatial Encoding and the Single-Scan Acquisition of High Definition MR Images in Inhomogeneous Fields. J Magn Reson, 2006. 181: p. 179-194; Shrot, Y. and L. Frydman, Spatially Encoded NMR and the Acquisition of 2D Magnetic Resonance Images with a Single Scan. J Magn Reson, 2005. 172: p. 179-190. Although this method makes use of a novel spatial-encoding scheme, it does not solve susceptibility problems arising from T2* decay throughout the echo train.
Dante fast imaging sequences have also been developed. These include, for example, BURST, DUFIS, and URGE. BURST and its variants are ultrafast imaging sequences that use the sidebands of the DANTE train to delineate pixels in the frequency-encoded direction. In BURST sequences an echo is formed and image reconstruction requires Fourier transformation (FT) in each dimension For a given pixel in BURST, the DANTE train excites a strip that is narrower than the pixel width. Several approaches have been proposed to improve the signal-to-noise ratio (SNR) of DANTE-based imaging methods. One method uses a frequency-modulated (FM) DANTE. Although FM-DANTE offers an improvement over the original BURST sequence in terms of SNR, intrapixel (voxel) phase cancellation occurs due to the quadratic phase of the excitation pulse.
Accordingly, it would be useful to have a technique with contiguous slices where no information is thus lost in gaps between slices. It would be further be useful to have a method of sequential magnetization that avoids susceptibility artifacts and blurring due to T2 (or T2*) decay, experiences a uniform flip angle (for example, a 90° flip angle) across the object, and, the relative transverse magnetization (Mxy) is uniform in the time-encoded direction (i.e., no gaps between pixels).